1. Field of the Invention
The invention concerns a method for optical data storage with high density, wherein there are employed as a data carrying medium a number of flat thin memory components in the form of cards or disks, wherein each component comprises one or more information carrying layers, wherein each memory component has one or more data carrying areas which are including one or more of the information carrying layers and adapted for optical data storage, and wherein the method comprises arranging two or more of the memory components in a stack, thus enabling each individual memory component to be maneuvered in relation to the other memory components by means of a maneuvering device. The invention also concerns a data carrying medium for use with the method for optical data storage with high density, wherein the data carrying medium comprises a number of flat, thin memory components in the form of cards or disks, wherein each memory component comprises one or more information carrying layers, and wherein each memory component comprises one or more data carrying areas which are including one or more of the information carrying layers and being adapted for optical storage of data.
2. Description of the Prior Art
In to-day's digital, optical data storage media information is stored in a flat layer on a disk, a card or tape. Circular disks or a rectangular card format are normally used. Even though the data density per area unit in the storage layer is very high, the effective volumetric storage density is not correspondingly high. Each disk or each card has only one or two data storage layers at the most and is typically 0.7-2 mm thick in order to provide the necessary rigidity and flatness for write/read operations. Moreover, when such media are placed in a protective cassette, there is a further increase in the volume. Even in the case of so-called "floppy" optical Bernoulli disks which have recently been proposed, the volumetric density is limited by the still substantial volume of the actual floppy disk's substrate, as well as the requirement for a protective housing or sleeve to cover the disk. The limiting effect of the packing on the volumetric density in connection with filing is particularly obvious in libraries which store CD cassettes.
The information carrying layer on present optical data storage media is thin, typically much less than 1 .mu.m. Thus only a small fraction of the volume of the disk or card is used directly for the data storage process. This is also the case even if the protective housing or sleeve is disregarded.
If it were possible to use the entire volume in a typical disk or a card for optical data storage, with a density which corresponded to that which at present can be achieved per area unit, this would be of substantial importance. By extrapolating from an area density which for example corresponds to a bit spacing of 1 .mu.m, the volumetric density could be 10.sup.9 bit/mm.sup.3. In a number of countries considerable efforts have been made to achieve digital, optical data storage within the volume of a data medium. A number of different methods have been proposed for this purpose:
(a) Holographic methods, in which refraction index changes are coded and read in the bulk material; PA1 (b) three-dimensional positioning of bit points in a volume-filling pattern within a bulk material, selective positioning being, achieved by means of sharp focusing of a laser beam, a non-linear response in the medium or an excitation on several wavelengths, PA1 (c) stacking of a number of flat information carrying layers close to one another in a uniform structure which may be a simple bulk material or a number of thin substrates which are attached to one another in order to form a sandwich structure. PA1 (1) A sharply focused laser beam with small depth of field can address a number of information carrying layers at different depths under the surface of a disk or a card by moving the focal point along the laser beam's axis, by analogy with confocal microscopy. At each depth level a thin optical storage layer must be provided in the disk's bulk material, e.g. by forming a dedicated plane structure. The absorbing, reflecting and transmitting properties for each layer must be carefully adapted and controlled in order to avoid crosstalk and covering up the deeper-lying layers. At present it is not clear to what extent these requirements limit the number of layers which can be used in practice. It appears to be obvious that the very stringent requirements regarding the flatness and optical quality of the disk or card cancel out some of the benefits which are obtained with a monolithic structure. PA1 (2) IBM recently presented a solution which resembles that which is described in the above example, but with a composite disk consisting of a number of thin disks which are joined together to form a sandwich structure, and each of which is equipped with an information carrying layer. Once again the optical properties of each layer must be carefully matched in order to avoid crosstalk and covering up. According to IBM it should be possible to stack up to ten layers in this manner. However, the composite disk is relatively thick and does not appear to be well adapted to the market requirements for inexpensive data carriers in a compact and practical format.
Of these above-mentioned methods, (a) and (b) appear to have the potential for the highest volumetric data density, but at the present time they are far from being capable of practical implementation, not least in equipment which should not be too expensive. In connection with the method described under (c), several different techniques are the subject of research and development, cf. the examples mentioned in the following. Even though these techniques appear to have the potential to increase data capacity on each disk or card up to tenfold, they all suffer from the drawback that the carrying substrate has to be manufactured within very strict optical and mechanical tolerances. In addition to the fact that this males for increased costs, there will also be a good chance that the individual component will be of a substantial thickness. The following examples of the state of the art and development trends will now be briefly discussed.
Finally it may as further examples of prior art be referred to that from GB patent No. 1 184 657 there is known a data storage medium with a number of photographic data storage elements in form of planar photographic transparencies which are displaced to selected positions for addressing and from EP-A1-0 293 495 an optical memory device with optical data carriers, for instance in form of cylinders which are guided in a container with an optical transparent window and therein can be displaced in translation and rotation for addressing from an optical write/read device.
Further there is from U.S. Pat. No. 3,800,942 known an information retrieval device comprising a multiplicity of jackets each for containing an information card carrying picture information, a plurality of magazines each for containing a plurality of such jackets, a drum for supporting a plurality of such magazines in juxtaposition and a means for transporting the magazines on the drum at random to a selector means for thereby to retrieve a desired information card from a desired magazine transported to the selector means. The device is mechanically complicated, as each magazine must be transported to the selector means before the information card can be retrieved.
As further examples of prior art there may be referred to that from GB patent No. 1,184,657 there is known a data storage medium with a number of photographic data storage elements in form of planar photographic transparencies which are displaced to selected positions for addressing and from EP-A1-0 293 495 an optical memory device with optical data carriers, for instance in form of cylinders which are guided in a container with an optical transparent window and therein can be displaced in translation and rotation for addressing from an optical write/read device.
Finally, it should also be mentioned that players for playback of a gramophone records and CD records may be equipped with facilities for automatic record changing. This is usually done by arranging the records in a stack-like configuration and extracting a record from the stack-like configuration for playback. The stack-like configuration may be contained in a magazine, as in the case in juke boxes and some CD players. However, in order to be manoeuvred and transported to the playback device, the records in the stack are physically separated and magazines for playback devices of this kind tend to be quite voluminous. However, International Published Patent application WO88/03694 discloses a record player for CD records stacked in a magazine and wherein a record is only partially extracted from the magazine in order to engage with drive mechanism for playback. The same principle could easily well be applicable in a CD ROM player with a magazine for CD ROMs. The drive and read means will in any case be located outside the magazine as the record must be freely rotated in the reading operation.
Further it should also be mentioned that players for playback of a gramophone records and CD records may be equipped with facilities for automatic record changing. This is usually done by arranging the records in a stack-like configuration and extracting a record from the stack-like configuration for playback. The stack-like configuration may be contained in a magazine, as is the case in juke boxes and some CD players. However, in order to be maneuvered and transported to the playback device, the records in the stack are physically separated and magazines for playback devices of this kind tend to be quite voluminous. However, International Published Patent application WO88/03694 discloses a record player for CD records stacked in a magazine and wherein a record is only partially extracted from the magazine in order to engage with drive mechanism for playback. The same principle could easily well be applicable in a CD ROM player with a magazine for CD ROMs. The drive and read means will in any case be located outside the magazine as the record must be freely rotated in the reading operation.